(a) Technical Field of Invention
This invention relates to an inkable sheet, and, in particular, to a sheet suitable for use with a mechanical printing assembly, such as an ink jet printer or a pen plotter.
(b) Background of the Art
With the recent proliferation of micro-computers and colour monitors there has been a massive growth in the amount of information available for display in colour. Presentation of such information has created a demand for hard copy, for example--on paper sheets, but increasingly on transparent polymeric films which are capable of serving as imaged transparencies for viewing in a transmission mode. Preparation of the desired hard copy is conveniently effected by, for example, an ink jet printer or a pen plotter, using an aqueous or an aqueous-organic solvent-based ink.
Ink jet printing is already established as a technique for printing variable information such as address labels, multi-colour graphics, and the like. A simple form of ink jet printer comprises a capillary tube coupled to an ink reservoir and a piezo-electric element which, on application of a voltage pulse, ejects an ink droplet from the capillary tube at high velocity (e.g. up to 20 ms.sup.-1) onto an ink-receptive sheet. Movement of the ink jet may be computer controlled, and new characters may therefore be formed and printed at electronic speeds. To derive advantage from this high speed operating capability requires the use of an ink-receptive sheet which will quickly absorb the high velocity ink droplet without blotting or bleeding. Although plastics sheets may be employed, these generally tend to exhibit inferior ink absorption and retention characteristics. In particular, drying of an applied ink pattern is slow, and immediate handling of a freshly imaged sheet is therefore prevented.
Pen plotter assemblies are extensively used in drawing offices, and particularly in the generation of computer aided designs. The advent of polymeric recording sheets has revealed that the formation thereon of inked images of acceptable quality usually requires the development of special, and expensive, pens. However, an inkable sheet according to the invention permits the use of a simple, inexpensive, fibre-tipped, aqueous based or hydrophilic ink, pen of the kind hitherto conventionally used with paper recording sheets.
(c) The Prior Art
Various recording sheets have been proposed for use with ink jet printers. In particular, U.S. Pat. No. 4,474,850 discloses an ink jet recording transparency said to be capable of being wetted by and absorbing coloured, water-soluble inks to provide high density images which are smear resistant, the transparency comprising:
(a) a substantially transparent resinous support, such as a polyester or polyvinyl chloride film, and PA1 (b) a substantially clear coating which includes a carboxylated, high molecular weight polymer or copolymer or salts thereof.
The carboxylated polymer or copolymer coating particularly comprises monomers of acrylic or methacrylic acid and esters thereof, vinyl acetates or styrenated acrylics, and usually has a molecular weight of from about 50,000 to 1 million. We have observed that an inked pattern applied to such a film transparency is relatively slow to dry, and that such transparencies are particularly susceptible to curling whereby a pattern applied thereto appears distorted when viewed as a transmission image.
We have now devised an inkable sheet which is particularly suitable as a recording sheet for use with a mechanical printing assembly, such as an ink jet printer or a pen plotter, the sheet exhibiting an improved rate of ink absorption, a reduced tendency to curl, and an improved resistance to moisture.